Digital cameras including digital still cameras (i.e., static image cameras) and digital video cameras (i.e., moving image cameras) are provided with an auto-focus function for automatically placing the focus point of the lens on a camera subject. Focusing is achieved by using the peak value or the like of a second-order differential of a captured image as a contrast value and by adjusting the distance between the lens and the imaging sensor to maximize the contrast value. When a camera subject on which the focus point is placed moves, for example, such movement may cause the loss of focus. In such a case, whether the focus point should be brought closer to the lens or moved farther away from the lens relative to the current focus position is determined based on the direction in which the previous auto focus control has moved the focus position. Namely, if the immediately preceding auto focus control has moved the focus position closer to the lens, the current auto focus control also brings the focus position closer to the lens by a distance corresponding to one control step. If the immediately preceding auto focus control has moved the focus position away from the lens, the current auto focus control also moves the focus position away from the lens by a distance corresponding to one control step. If the movement of the focus position corresponding to one control step results in an increase in the contrast value, the focus position continues to be moved in the same direction until the position is found at which the contrast value becomes the maximum. If the movement of the focus position corresponding to one control step results in a decrease in the contrast value, the focus position is moved in the opposite direction until the position is found at which the contrast value becomes the maximum.
In the continuous AF which continuously performs auto focusing as described above, the direction in which the focus point is moved by a distance corresponding to one control step upon losing focus may be an incorrect direction, and the occurrence of such a mistake is not rare. The focus position is controlled by mechanically adjusting the distance between the lens and the imaging face of the imaging device. It is thus difficult to achieve a high-speed operation which moves the lens in one direction immediately after moving it in the opposite direction that has turned out be a wrong direction. When the direction of initial movement is incorrect, the time required to achieve focusing becomes lengthy.